Lead containing glazes and enamels for glass substrates are well-known in the ceramic art. A glaze is typically thought of as a clear glass coating, whereas an enamel is a glaze which contains a pigment or other particles in suspension. Such glazes and enamels often are applied to glass surfaces in the form of a paste, the paste containing finely ground particles of glassy material, commonly called "frit" and a vehicle. After application of the glaze or enamel to the glass surface by silk screening or other techniques, the glass is fired to volatilize and/or thermally decompose and drive off the vehicle, fuse the frit, and bond the glaze or enamel to the glass surface.
In the past, the use of lead oxide in the frit served to lower the melting point of the frit. Recently, efforts have been made to retain the low melt temperature of the glazes and enamels without the use of lead.
An example of a prior art lead-free glass enamel may be found in Roberts, U.S. Pat. No. 4,859,637. Roberts discloses a lead-free glass enamel including a frit having in weight percent 19-45% SiO.sub.2, 20-33% B.sub.2 O.sub.3, 8-15% alkali metal oxide, 0-35% ZnO, 0-10% ZrO.sub.2, 0-6% Al.sub.2 O.sub.3, 0-3% BaO, 0-8% P.sub.2 O.sub.5, 0-6% La.sub.2 O.sub.3, 2-10% F.sub.2, and 2-25% of a U.V. additive.
Another example of a prior art lead-free glass enamel may be found in Francel et al., U.S. Pat. No. 4,537,862. Francel et al. discloses a glass enamel including a frit comprising by weight 25-32% SiO.sub.2, 25-32% B.sub.2 O.sub.3, 10-18% ZrO.sub.2, 0-9% Na.sub.2 O, 0-5% K.sub.2 O, 0-9% Li.sub.2 O, 0-2% SnO.sub.2, 0-3% F.sub.2, 0-3% CaO, 0-6% SrO, and 10-18% rare earth oxide wherein the ratio of ZrO.sub.2 to rare earth oxide is about 1:1 to 1.4:1, the amount of ZrO.sub.2 plus rare earth oxide is about 20% to 33% and the amount of Li.sub.2 O, K.sub.2 O and Na.sub.2 O is about 1 to 10.